1. Field of the Invention
The present invention relates to an electron-beam excitation laser, and more particularly, to an electron-beam excitation laser emitting light distributed from the visible region to the ultraviolet region. An electron-beam excitation laser according to the present invention can be used as a light source for a laser beam printer, an optical recorder, optical communications, a pointer, a display, and so on.
2. Related Background Art
A laser consists commonly of a laser medium, a resonator including a reflector, and an excitation source. A laser medium obtains energy from an excitation source to emit light with a specific wavelength, so that a laser beam is generated in a state that allows amplification with gain. An optical resonator reflects light emitted by the laser medium to return it to the medium. The light is caused to reciprocate many times to be progressively amplified thereby cause laser oscillation. Lasers come in various types according to what they use as a laser medium: a gas laser which uses a He—Ne mixture, argon gas or the like; a solid laser which uses Nd:YAG, Ti:sapphire or the like; a dye laser which uses a dye; and a semiconductor laser which uses a semiconductor made of GaAlAs or the like. An optical excitation laser which uses light, a current injection laser which uses current, an electron-beam excitation laser which uses an electron beam. As excitation source, such lasers are known as: a light excitation laser which uses light; a current injection laser which uses current; and an electron beam excitation laser which uses electron beam. Resonators include a Fabry-Perot resonator, which uses a reflector, a prism, and a diffraction grating, and a ring resonator. Semiconductor lasers include those which use a cleavage plane or a multilayer film as a reflecting surface, distribution feedback (DFB) lasers, and distribution Bragg reflection (DBR) lasers.
A semiconductor laser is small and light, consumes a small amount of electric power, and offers high electricity-light conversion efficiency. To provide a higher-density optical recorder and a color display (refer to Japanese Patent Application Laid-Open No. 6-89075), a small laser is hoped for which emits light with a short wavelength, especially light distributed from the blue region to the ultraviolet region. For a reason of the exposure performance of a laser beam printer also, such a small laser is demanded. However, it is difficult to cause laser oscillation by current injection, using a wide-band-gap semiconductor because of its electrical characteristics, such as the fact that such a semiconductor cannot freely be doped. On the other hand, all types of direct-gap semiconductors, especially direct-gap semiconductors which are difficult to use for current injection lasers can be used for electron-beam excitation lasers. Because of this, direct-gap semiconductors are expected to be used for laser oscillation from the ultraviolet region to the infrared region.
A common electron-beam excitation laser will be described below. FIG. 17 shows the structure of a conventional electron-beam excitation laser. In the figure, a reference numeral 101 indicates a substrate; 102, a light emitter (active layer); and 103 and 104, reflectors. When electrons are emitted from an electron source, not shown, the light emitter 102 is excited, and the reflectors 103 and 104 serves as resonators, so that laser oscillation occurs, thus emitting a laser beam. In FIG. 17, a reference numeral 200 indicates a direction of electron emission, and a reference numeral 300 indicates a direction of laser beam emission. Such is also the case with other figures. Semiconductors containing a group II–VI group compound, such as ZnS, CdTe, or ZnSe, are mainly used for the active layer. As a light reflector, are used a substrate cleavage plane, a metal reflector made of Al or the like, dielectric multilayer film made of SiO2, TiO2, or the like, and multilayer reflector, that is, a combination of two compound semiconductors with different refractive indexes are used for the reflectors constituting a resonator.
A small electron-beam excitation laser has been reported which was designed using a spint type electron emitting diode and a CdTe/CdMnTe-based laser structure (Applied Physics Letters, Vol. 62, p. 796, 1993).
The following problems have prevented conventional electron-beam excitation lasers as described above from being brought into practical use:
(1) Conventional electron-beam excitation lasers are low in light emission efficiency and high in oscillation threshold value. Laser oscillation occurs over a wide wavelength region in various modes.
(2) When a thick reflecting layer is provided so that its side to which an electron beam is irradiated has sufficient light reflecting capability, an electron beam attenuates by the time it reaches the active layer, thus lowering light emission efficiency. To prevent such a reduction in light emission efficiency, electron-beam accelerating voltage must be increased.
(3) Because a laser structure (a semiconductor layer) is made by vapor-phase growth methods, such as MBE and CBE, the structure is costly, so that lasers using such a structure are difficult to offer at low cost.